Reacting a chrome-tanned hide first with a sulfonic acid syntan and then with methylol-melamine



Patented Sept. 19, 1950 REACTING A CHROME-TANNED HIDE FIRST WITH ASULFONI-C ACID. SYNTAN AND THEN WITH METHYLOL -MELAMINE William '0.Dawson, Stamford, Conn., assignor to American Cyanamid Company, NewYork, N. Y., a corporation of Maine No Drawing.

This invention relates to the production of I white leather by retanningchrome tanned leather with a Water-soluble methylol melamine. Theprincipal object of the invention is to obtain leather having greatlyimproved whiteness and light-resistance imparted by the methylolmelamine while maintaining the strength and other desirable propertiesof chrome tanned leather. A further object is the provision of a processthat is capable of making a high-grade white leather from skinny,spready hides that are unsuitable for making chrome white leather byother processes. Still further objects will appear from the fol owingdescriptions of preferred embodiments of the invention.

In my Patent No. 2,316,740 dated April 13, 1943, I have shown that hidesor skins that have been tanned or partially tanned with methylolmelamines may be treated with other tanning agents such as vegetabletans, chrome tannages, and iron tannages. The process of that patent isbased on the preliminary tannage of hides or skins with water-solublemethylol melamines, followed by applying the chrome, vegetable or irontannage on top of the methylol melamine.

My present invention, like my earlier patent above referred to, isdirected to a combined tannage of hides or skins with a basic chromiumsulfate taming agent and with methylol melamine, but constitutes animprovement thereover. I have now discovered that a white leathercomparable in feel and texture to the white leather produced by tanningskins with melamine-formaldehyde resins alone, as described in my PatentNo. 2,316,741, can be obtained by the procedures hereinafter described.This leather, in addition to its excellent white appearance and texture,has all of the strength and other desirable characteristics of chrometanned leather, and therefore constitutes a chrome leather of greatlyimproved surface appearance. Moreover, the melamine resin-treated chromeleather of my present invention has a substantially increased plumpnessand weight as compared with chrome leather prepared from the same stockbut containing no methylol melamine, and is therefore greatly enhancedin value.

Mv present invention, in its broadest aspects, consists in theapplication to chrome tanned leather ofan aromatic sulfonate tanningagent and awater-soluble melamine-formaldehyde condensation product. ormethylol melamine, in suitable amounts. The advantages which I obtain bythis treatment will be clear from the following discussion:

In the manufacture of chrome tanned leather by either the one-bath orthe two-bath process the substance of the hide or skin is caused tocombine with a basic chromium sulfate which converts it into leather.The chrome leather from this treatment is described in the tanneryApplication November 19, 1946, Serial No. 710,949

4 Claims. (01. 8-94.33)

. 2 a as "blue'stock, since it is light blue in color. When'whiteleatheris desired it is customary to bleach this blue stock with an aromaticsulfo nate tanning agent such as disulfodinaphthyl methane, obtained bycondensing about two mols of naphthalene-sulfonic acid with one mol offormaldehyde; or with the corresponding formaldehyde condensationproducts of sulfonated phenols or sulfonated cresols. These bleachingagents are believed to form 'a colorless complex with the chromepresent-in the leather and thus lighten its color; however, the bleachedleather still retains a bluish cast and is somewhat gray incol'or'rather than White. r -I have found that wateresolublemelamineformaldehyde condensation products, or meth: ylol melamines,will combine with the bleached, chrome tanned leather to form leatherwhich is both permanent and truly white in color. More-. over, theleather which -I obtain by impregnating the bleached chrome leatherwithmethylol melamine is light-resistant; i. e., the discolora-z tion uponcontinued exposure of the leather to ultra-violet light is much lessthan with ordi-e' nary chrome leather bleached with aromatic sulfonatetanning agents. Impregnation of the bleached chrome leather withmethylol mela; mine also causes an additional plumpingand increaseinweight, but without any of the difficulties' "caused by drawn grainthat are fre--- quently encountered in plumping,and the white leather soobtained canbe lubricated and softened by ordinary fat-liquoringtreatments with vegetable or animal oils which may be unsulfonated butare preferably sulfonated in character. .1'

One of the most remarkablefeatures of the process of my invention. isthe fact that it can be ..used to make a high-grade white leather fromskinny, spready sides that are normally unsuitable for makingchromewhite leather and haveheretofore required heavy vegetableretannage. The following is a'specific example of the application of theprocess of my invention to this type of skins; however, it will beunderstood that'the process isalso app icable without material :change.to skins of better quality, and the invention is not limited tothetreatment of skins of low r ade.

, 1,000 pounds of pickled sides of the above grade were placed in atanning drum together with gallons of water containing 50 pounds of salt(NaCl) and 15 gallons of 33.5% basic chrome liquor were added containingan amount of basic chromium sulfate equivalent to 2-2.5 pounds of sodiumbichromate per gallon. The chrome liquor was added in three feeds about20 minutes apart while rotating the drum, and the drumming was continuedfor a total of 3 hours. The liquor was then neutralized with sufficientsodium bicarbonate solution-until;theleather showed a heat resistance of200 F.

The chrome tanned sides were-placed in a tanning drum and washed 30minutes in water at temperatures up to l25-135 F. The water was changedand 3% of a disulfodinaphthylmethane tanning agent having a pH .of 6-8was added to the drum, based on the shaved weight of the sides, and thedrum was rotated 45 minutes, drained and washed 15 minutes at 130 F.,and again drained. Approximately 75-100% of water at 130 F. was thenintroduced into the drum and 3.5% of trimethylol melamine was added,based on the weight of the sides. The drum was then rotated for 30minutes, allowed to stand for one hour, rotated an additional 30minutes, and then stopped overnight. The following morning the drum wasrotated 15 minutes and the .leather was washed with water andfatliquored. of sulfonated vegetable oil and-1% of neats-foot oil wereused for the fat liquoring along with 4% of titanium dioxide, and thedrum was rotated 30 minutes. The leather was then horsed up and dried.

The leather prepared by this process was found to have gained /2 to 1ounce in weight, as compared with white chrome tanned leather from thesame stock to which no methylol melamine was added. The leather was verywhite, had a deep white cut showing good impregnation of the methylolmelamine, was full and tight, and had an unusually uniform break.

The water-soluble methylol melamines used in practicing my inventionshould remain water soluble in the presence of salt, and shouldtherefore be substantially unpolymerized. They may be prepared asdescribed in my issued patents referred to above, or by any othersuitable process. The trimethylol melamine used in the above example wasa spray-dried methylol melamine which was substantially unpolymerizedand not salt-sensitive. This product was prepared as follows:

A quantity of formaldehyde in the form of a 37% aqueous solution wasweighed, charged to a mixing vessel maintained at a temperature below 60'F., and adjusted with sodium hydroxide to a pH of 8.0. Micropulverizedmelamine was added to the formaldehyde solution, with agitation, in anamount sufficient to bring the molecular ratio of formaldehyde tomelamine to 3.611, care being taken to maintain the temperaturesufficiently low to prevent reaction. This unreacted slurry wasimmediately pumped under pressure through a heat exchanger at atemperature of 220 F., at such a rate that it took approximately 2minutes for a given quantity of the slurry to pass therethrough. Thematerial was then pumped through a cooling heat exchanger, which rapidlycooled the syrupy product to approximately 120 F. From the cooling heatexchanger the product was introduced into a spray drier and spraydriedat a gas temperature of 450 F. The process was continuous from one stepinto the other, as all the individual units were interconnected forcontinuous flow. The product had the following analysis:

1.4% moisture 36.5% nitrogen 42.2 total formaldehyde 2.9% freeformaldehyde 3.0-3.2 molecular ratio of formaldehyde to melamine pH of7.8

A fusion temperature of 81.5 C.

A solubility in water at 25 C of 25 parts per 100 parts of water, and 50parts per 100 at 75 Q.

Infinitely -water dilutable without producing hydrophobidity The amountof synthetic tanning agent may vary from about 1% to about 5%, based onthe weight of the leather. Usually hides of better grade than thosedescribed in the above example will require smaller quantities than the3.5% that was actually used, about 12.5% being sufficient in most cases.Although a neutralized synthetic tanning agent is preferred, thearomatic sulfonate may be alkaline or acid.

The water-soluble methylol melamine may be used in amounts of about 1%to about 5%, based on the weight of the chrome tanned leather, the exactamount depending on the degree of whiteness and fullness desired.Although the methylol melamine is usually applied after the aromaticsulfonate tanning agent, it may be applied simultaneously therewith ifdesired; in fact the aromatic sulflonate may be applied after themethylol melamine in some cases.

From the foregoing description it is apparent that the process of myinvention can be applied either to hides such as cowhide or to so-calledskins such as calfskins, gloat skins, etc. In the appended claims theterm skins is used to designate either hides or skins, both of which maybe treated with equal facility by the process of my invention.

What I claim is:

l. A method of obtaining a light-resistant white chrome tanned leatherwhich comprises re acting fully tanned chrome tanned blue stock withabout 1-5% of its weight of an aromatic sul- Ionate tanning agent andsubsequently reacting the aromatic sulfonate treated stock with about2-5% of its weight of a water-soluble methylol melamine.

2. A method of obtaining a light-resistant white chrome tanned leatherwhich comprises reacting fully tanned chrome tanned blue stock withabout 1-5% of its weight of a disulfodinaphthylmethane tanning agent andsubsee quentl reacting the di-sulfodinaphthylmethane treated stock withabout 2-5'% of its weight of water-soluble methylol melamine.

3. A method of producing chrome-tanned leather lof improved whitenes andlight-resistance from skins so inferior in quality as to be normallyunsuitable for making chrometanned leather which comprises firstsubjecting said skins to a full chrome tannage, then reacting theresulting chrome-tanned leather of in.- ferior grade with about 1-5% ofits weight of .a formaldehyde condensation product of an arcmaticsulfonic acid, and then reacting it with about 1-5% of its weight of awater-soluble methylol melamine.

4. A method according to claim 3 in which the aromatic sulfonicacid-formaldehyde condensation product is a .disulfodinaphthyl methane,

WILLIAM O. DAWSON,

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. A METHOD OF OBTAINING A LIGHT-RESISTANT WHITE CHROME TANNED LEATHERWHICH COMPRISES REACTING FULLY TANNED CHROME TANNED BLUE STOCK WITHABOUT 1-5% OF ITS WEIGHT OF AN AROMATIC SULFONATE TANNING AGENT ANDSUBSEQUENTLY REACTING THE AROMATIC SULFONATE TREATED STOCK WITH ABOUT2-5% OF ITS WEIGHT OF A WATER-SOLUBLE METHYLOL MELAMINE.